A fuel vapor treatment system that discharges a fuel vapor in a fuel tank into an intake passage of an internal combustion engine and treats the fuel vapor has been known. In recent years, regulations of the fuel vapor leaked from an inside of the fuel tank to an outside become strict. In particular, according to a standard of the United States Environmental Protection Agency (EPA) and the United States California Air Resources Board (CARB), it is required to detect the leakage of the fuel vapor from a fine hole of the fuel tank.
An fuel vapor treatment system disclosed in JP 2006-177199 A includes a tank passage that connects a fuel tank and a canister, a purge passage that connects the canister and an intake passage, an atmospheric passage that connects the canister and the atmosphere, a purge valve that can open and close the purge passage, an atmospheric valve that can open and close the atmospheric passage, and a pressure sensor that can detect pressure in the purge passage. After an internal combustion engine is stopped, the fuel vapor treatment system closes the atmospheric passage and the purge passage so that a leak abnormality that is “an abnormality of leakage of an fuel vapor from the fuel tank, the tank passage and the like to an outside” can be detected based on a signal from the pressure sensor after a predetermined time has elapsed.
In the fuel vapor treatment system described above, the liquefied fuel vapor or a foreign substance is retained in the tank passage and thereby the tank passage might be clogged. However, in the fuel vapor treatment system, the pressure sensor is arranged in the purge passage, namely on an opposite side to the fuel tank with respect to the tank passage. Thus, when the tank passage is clogged, the clog cannot be detected by the pressure sensor. When the tank passage is clogged, the pressure in the fuel tank is increased and a valve body, which closes a fuel supply port or the like, is opened, broken or the like, and therefore the fuel vapor might be leaked from the fuel supply port to the outside.